Cellular communications systems are well known in the art. In a typical cellular communications system, a geographic area is divided into a series of regions that are referred to as “cells,” and each cell is served by a “macrocell” base station. Each cell may, for example, have an area on the order of 1-50 km2, with the cell size depending upon, among other things, the terrain and population density. The base station may include baseband equipment, radios and antennas that are configured to provide two-way radio frequency (“RF”) communications with fixed and mobile subscribers (“users”) that are positioned throughout the cell. The base station antennas are often mounted on a tower or other raised structure, with the radiation beam (“antenna beam”) that is generated by each antenna directed outwardly to serve the entire cell or a portion (“sector”) thereof. Typically, a base station antenna includes one or more phase-controlled arrays of radiating elements, with the radiating elements arranged in one or more vertical columns when the antenna is mounted for use. Herein, “vertical” refers to a direction that is generally perpendicular relative to the plane defined by the horizon.
In order to increase capacity, cellular operators have been deploying so-called “small cell” base stations. A small cell base station refers to a lower power base station that may operate in the licensed and/or unlicensed spectrum that serves a much smaller area than a typical macrocell base station. Herein, the term “small cell” is used broadly to refer to base stations that serve smaller areas than conventional macrocell base stations, and thus the term “small cell” encompasses small cell, microcell, picocell and other base stations that serve small geographic regions. Small cell base stations may be used, for example, to provide cellular coverage to high traffic areas within a macrocell, which allows the macrocell base station to offload much or all of the traffic in the vicinity of the small cell to the small cell base station. Small cell base stations may be particularly effective in Long Term Evolution (“LTE”) cellular networks in efficiently using the available frequency spectrum to maximize network capacity at a reasonable cost.
FIG. 1 is a schematic diagram of a conventional small cell base station 10. The base station 10 includes an antenna 20 that may be mounted on a raised structure 30. In the depicted embodiment, the structure 30 is a small antenna tower, but it will be appreciated that a wide variety of mounting locations may be used including, for example, utility poles, buildings, water towers and the like. Typically, the antenna 20 of a small cell base station is designed to have an omnidirectional antenna pattern in the azimuth plane, meaning that the antenna beam generated by the antenna 20 may extend through a full 360 degree circle in the azimuth plane, and may have a suitable beamwidth (e.g., 10-30 degrees) in the elevation plane. The antenna beam may be slightly down-tilted in the elevation plane (which may be a physical or electronic downtilt) to reduce spill-over of the antenna beam of the small cell base station antenna into regions that are outside the small cell and also for reducing interference between the small cell base station and the overlaid macrocell base station.
The small cell base station 10 further includes base station equipment such as one or more baseband units 40 and radios 42. While the radio 42 is shown as being co-located with the baseband equipment 40 at the bottom of the antenna tower 30, it will be appreciated that in other cases the radio 42 may be a remote radio head that is mounted on the antenna tower 30 adjacent the antenna 20. As is known to those of skill in the art, the baseband unit 40 may receive data from another source such as, for example, a backhaul network (not shown) and may process this data and provide a data stream to the radio 42. The radio 42 may generate RF signals that include the data encoded therein and may amplify and deliver these RF signals to the antenna 20 for transmission via a cabling connection 44. It will also be appreciated that the small cell base station 10 of FIG. 1 may typically include various other equipment (not shown) such as, for example, a power supply, back-up batteries, a power bus, controllers and the like.